This invention relates to analog signal processing, and more particularly to radio receivers with clarification control means. This invention further relates to the transceiver and components thereof described and claimed in the following U.S. Patent Applications filed of even date with and assigned to the assignee of the present invention: U.S. Ser. No. 791,611 entitled "A Digitally Transmitting Transceiver" by Edward R. Caudel and William R. Wilson; U.S. Ser. No. 791,253 entitled "A Transceiver Capable of Sensing A Clear Channel" by Jerry D. Merryman, Michael J. Cochran and Edward R. Caudel; U.S. Ser. No. 791,449 entitled "An Automatically Clarifying Radio Receiver" by Michael J. Cochran and Edward R. Caudel; U.S. Ser. No. 791,254 entitled "A Computer Controlled Radio System" by Michael J. Cochran and Edward R. Caudel; U.S. Ser. No. 791,450 entitled "A Transceiver With Only One Reference Frequency" by Michael J. Cochran; U.S. Serial No. 791,614 entitled "A Charge Transfer Device Radio System" by Michael J. Cochran; U.S. Ser. No. 791,265 entitled "A Signal Strength Measuring Transceiver" by Edward R. Caudel; U.S. Ser. No. 791,256 entitled "A Highly Selective Programmable Filter Module" by Michael J. Cochran and Edward R. Caudel; U.S. Ser. No. 791,616 entitled "A Dual Processor Transceiver" by Edward R. Caudel, William R. Wilson and Thomas E. Merrow; U.S. Ser. No. 791,264 entitled "An Electronic Phase Detector Circuit" by Michael J. Cochran. Radio systems receive radiated electronic input signals comprised of a plurality of non-overlapping frequency bands, filter a selectable band from the plurality, frequency shift the filtered band from radio frequencies to a lower frequency, and convert the selected band to audible sounds. The input signals may be amplitude modulated (AM), or single sideband (SSB) signals as an example. Antenna means receive the radiated input signals. Filtering devices are included in the radio system to select one of the bands from the plurality. Mixing devices are included to frequency shift the selected band. And demodulator devices are included to demodulate the selected band.
In the past, single sideband signals have been particularly troublesome to receive with clarity. The problem arises because sideband signals have no carrier to lock onto to aide in demodulation. And demodulation is made more difficult because transmitters which send single sideband signals are not required to align the signals with a fixed reference frequency, but only are required to place the sideband signals somewhere within a frequency channel. Thus, the receiving system is required to demodulate the sideband signals regardless of where they lie within their respective channel. This requirement is known in the art as the clarifying function of a receiver.
Prior art receivers varied the frequency of mixing clock signals in order to compensate for variations of where the sideband signals actually resided in its assigned channel. That is, mixing devices mixed the sideband signals with either a relatively high frequency or relatively low frequency dependent upon whether the sideband signal laid in the upper frequency portion or the lower frequency portion of its channel. By comparison, one embodiment of the radio system herein described varies the clocking frequency of a transversal filter to clarify the sideband signal being received. The transversal filter has a plurality of spaced apart passbands which define the frequency range of its output signals. Both the center frequency and the bandwidth of the passbands are dependent upon the clocking frequency of the filter. Thus, a novel feature of the invention is that the mixing clock signals are not varied to perform a clarifying function, but are varied solely for the purpose of channel selection. As a result, the clocking means for generating the mixing clock signals need only be capable of generating frequencies in increments of the channel spacing. Conversely, the clocking means for generating the filter clock signals must be capable of generating frequencies which are adjustable by small increments about a predetermined nominal frequency.
A sampling demodulator couples to the output of the charge transfer device filter for demodulating signals from the lowest frequency passband of the filter. The frequency of the demodulator sampling clock is not dependent upon where the sideband signals lie within their channel. This is because the lowest frequency passband of the filter will not move appreciably in response to the clocking frequency of the filter, whereas higher order passbands of the filter will be shifted to precisely align with the frequencies of the selected band regardless of where it lies within its respective channel.
Accordingly, it is one object of the invention to provide an improved single sideband radio receiver.
It is another object of the invention to provide a radio system capable of clarifying single sideband signals without varying the mixing frequency of any mixing device.
Another object of the invention is to provide a radio system having a charge transfer device filter for clarifying single sideband signals.